Add values to dict of list in Python?

Question

I have this kind of dictionary of lists, initially empty:

d = dict()

Now the use case is to simply add a value to list under a key, which might be a new or an existing key. I think we have to do it like this:

if key not in d:
    d[key] = list()
d[key].append(value)

This seems awfully tedious and error-prone, needing to write multiple lines (with copy-paste or helper function). Is there a more convenient way?

If there isn't "only one way to do it", and you know it, you can also answer that, and maybe suggest alternative ways to accomplish above, even if they aren't necessarily better.

I looked for duplicate, didn't find, but perhaps I just didn't know to use right keywords.

There's the .setdefault method for doing this sort of thing. — PM 2Ring
– PM 2Ring, Commented Sep 14, 2018 at 17:36

2080 · Accepted Answer · 2018-09-14 16:45:09Z

18

What you want is called a defaultdict, as available in the collections library:

Python2.7: https://docs.python.org/2/library/collections.html#defaultdict-examples

Python3.7: https://docs.python.org/3/library/collections.html#collections.defaultdict

Example:
>>> from collections import defaultdict
>>> s = [('yellow', 1), ('blue', 2), ('yellow', 3), ('blue', 4), ('red', 1)]
>>> d = defaultdict(list)
>>> for k, v in s:
...     d[k].append(v)
...
>>> sorted(d.items())
[('blue', [2, 4]), ('red', [1]), ('yellow', [1, 3])]

answered Sep 14, 2018 at 16:45

2080

1,4372 gold badges18 silver badges46 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Superior · Accepted Answer · 2019-08-24 17:32:28Z

12

d[key] = d.get(key, []) + [value]

to explain
d.get method returns value under the key key and if there is no such key, returns optional argument (second), in this case [] (empty list)

then you will get the list (empty or not) and than you add list [value] to it. this can be also done by .append(value) instead of + [value]

having that list, you set it as the new value to that key

e.g.

d = {1: [1, 2]}
d[1] = d.get(1, []) + [3]
# d == {1: [1, 2, 3]}

d[17] = d.get(17, []) + [8]
# d == {1: [1, 2, 3], 17: [8]}

edited Aug 24, 2019 at 17:32

answered Sep 14, 2018 at 16:57

Superior

8855 silver badges18 bronze badges

2 Comments

user2390182 Over a year ago

Why not just d.setdefault(key, []).append(value)?

Superior Over a year ago

@schwobaseggl that works, but only for mutable types, it would fail for example dict(str:int)

Dylan · Accepted Answer · 2018-09-14 16:56:12Z

2

d[key] = list() if (key not in d) else d[key].append(value)

Uses the ternary operators. And looks somewhat cleaner, however it is still almost identical to your scenario.

I apologize, I wasn't thinking earlier and used standard ternary operators. I changed them to Python ternary operators.

edited Sep 14, 2018 at 16:56

answered Sep 14, 2018 at 16:43

Dylan

1,40512 silver badges22 bronze badges

3 Comments

hyde Over a year ago

Python does not have ?: operator, does it...? There is the exp1 if cond else exp2 syntax though, you could rewrite the answer using that.

Dylan Over a year ago

@hyde that is my bad. I used the wrong ternary operators.

hyde Over a year ago

I think this doesn't work, still... d[key] = [value] if key not in d else d[key] + [value] should work and cover both cases correctly.

Collectives™ on Stack Overflow

Add values to dict of list in Python?

3 Answers 3

Comments

2 Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

2 Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related